Travellers heading to the UK have suffered delays of up to six hours at the Port of Calais.
Ferry operators DFDS and P&O Ferries apologised for the long wait times over the weekend, the last before the start of the new school year.
Queues were back to normal again on Monday, according to DFDS.
In a tweet on Monday, it advised travellers to “allow plenty of time to complete the check in process at the port”.
P&O Ferries apologised for the wait times at Calais on Saturday, saying they were due to “queues at border control”.
“We have put on an additional sailing to accommodate all those that have been stuck in queues,” it said in a tweet.
DFDS told passengers it was “expecting long wait times to complete controls.
“Please take this into consideration when travelling to the port, factoring in sufficient breaks for food and water before arriving.”
Operators said queues were five hours long.
But some passengers complained they had been stuck for up to six hours.
A spokesperson for the Port of Calais said: “Calais was affected by ‘the perfect storm’ of summer volumes in combination with post-Brexit border checks, causing six hours of queuing.
“We worked together with partners to reduce the queues as quickly as we could.”
Passengers took to social media to complain about the queues.
“Huge snarl-up at Calais. No queues before DFDS, no queues for French Border Control, but have now spent over an hour queuing at UK Border Control,” wrote one traveller.
“Hundreds of checked-in cars missing their ferries. Don’t know what lane to go to when we’re through as the ship has sailed!”
Passports have been subject to more detailed checks since the UK left the EU, which means border control officers need longer to examine each document.
They include ensuring a person has not been in the EU for more than 90 of the past 180 days, unless they have a visa or resident permit for the area.